When, Where, and How Big? Solving the Mysteries of Mudslides

A house damaged by rainstorm-triggered debris flows in Mullally Canyon, California, on February 6, 2010. The drainage basin above this home was burned the previous summer by the Station Fire, the largest fire in the history of Los Angeles County. (Photo: Susan Cannon/USGS)

Fierce storms pummeled the Pacific Northwest a couple of weeks ago, dumping 15 inches of rain in two days and causing a water-soaked hillside north of Portland to collapse onto Interstate 5. Thousands of motorists were stranded on the highway, the main artery connecting Washington and Oregon. Fortunately, no one was killed.

Parched California could use the heavy rainfall El Niño is predicted to deliver this winter. The recent muddy mess in Oregon, however, is a reminder of how drought-quenching moisture can also trigger devastating landslides, wreaking further havoc on a state already suffering from wildfires, sinking lands, and wildlife die-offs. Of particular concern is a specific type of landslide: mudslides, also called “debris flows,” which are set off by rain. So while El Niño storms are still weeks away, agencies are already keeping their eyes—and instruments—on vulnerable areas in an effort to give people a heads-up if a mudslide is likely.

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Bob Haus, a public information officer for the California Department of Transportation, says his agency is going above and beyond its usual preparations in anticipation of what could be a busy (and muddy) season. Employees are clearing culverts and dams of debris and scouting recently burned hillsides for weak points. Rock-climbing crews are investigating the integrity of hard-to-access stretches near homes and roads. “Its not a matter of if, but when,” says Haus of California’s next mudslide—an ominous prediction. Mudslides have been clocked at speeds of up to 200 miles per hour.

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Strong El Niño storms in the winters of 1982–83 and 1997–98 triggered numerous catastrophic landslides throughout the state. In January 1982, heavy rainfall over a 32-hour period set off more than 18,000 debris flows in the San Francisco Bay Area, killing 14 people and damaging more than 100 homes.

“It was tragic, but a bunch of research came out of it,” says Brian Collins, an engineer and landslide expert with the United States Geological Survey in Menlo Park, California. “We learned that sandier soils, rather than clay soils with small particles, tended to cause debris flows that careened out into neighborhoods.”

Two researchers at a monitoring site in the Bay Area. (Photo: USGS)

Today, Collins is building on that knowledge to gain an even better understanding of the conditions that make landslides likely. In the Bay Area, at least 10 to 12 inches of rain will typically fall before a mudslide occurs, but exactly how much additional precipitation will cause the ground to let loose depends on many other factors, such as soil type and when it last stormed. To get a handle on those variables, Collins installed instruments on four Bay Area hillsides in 2010 to record the amount of water in the ground, the saturation point of the soil, and how much additional rainfall is necessary to trigger a slide.

“To get to the point where we are comfortable saying, ‘These are the conditions that cause landsliding’ will take multiple storms, multiple events, multiple years,” he says.

The recent prolonged drought hasn’t helped. In the five years he’s monitored the sites, Collins has gathered little data. “Maybe we’ll get lucky this winter,” he says, quickly adding, “We don’t want landslides to happen, but if they do, we’ll learn a lot.”

Ultimately, the aim is to provide his findings to the National Weather Service, so it can combine the information with its forecasts and observed rainfall rates to issue early warnings on landslides, as it does for tornados and severe thunderstorms.

The NWS actually already issues warnings about a certain type of landslide: debris flows that occur in areas scorched by wildfire, where the barren ground is unable to absorb water and mudslides can form quickly. Collins’ colleagues at the USGS in Colorado, Jason Kean and Dennis Staley, study these post-fire debris flows and provide estimated rainfall thresholds for burn areas to the NWS.

To do that, Kean and Staley first identify the likeliest danger zones—areas that are steep, significantly burned, and often near urban areas like Los Angeles. At each flagged spot, the researchers deploy instruments like rain gauges and video cameras. They also deploy geophones, which measure the vibrations caused by debris flows, note the exact time a mudslide begins, and may soon provide other information, like the volume and concentration of a moving mud river.

“Basically, we calculate that when it starts raining so hard after X amount of time, then you can expect trouble,” Kean says.

This past summer, Kean and Staley were shocked to see just how fast a July rainstorm in a burned area of Orange County triggered a mudslide. “This wall of mud zoomed by within a few minutes of rainfall,” Kean says. “Honestly, before we started measuring, I didn’t realize they happened that fast.”

(Map: USGS)

All three landslide experts emphasize the importance of being prepared and heeding warnings. “There’s not much you can do to stop a 20-foot wall of mud,” says Staley, who along with Collins narrowly escaped being swept up in the line of duty. Kean had long wanted to see a landslide in person—that is, until his colleagues had their close call and had to run for their lives. “After that,” he says, “I figured the best way to watch one was with geophones, a lot of video, and a bag of popcorn.”

For residents of northern and Southern California, the USGS Landslide Hazard program offers tips on what to watch out for and expect this winter. To see clips of mudflows, check out the USGS’s footage here. Popcorn optional.

This story originally appeared on onEarth as “Slip-Sliding Away” and is re-published here under a Creative Commons license.